Chief witness Mr O'Brien took the stand yesterday. Earlier this week, the prosecuting counsel told the jury that Mr O'Brien was "an accomplice" and "implicated in this offence".

"He's up to his shoulders, if not his neck, in this," he said. "You do have to look very carefully at his evidence," Patrick Gageby told the jury on Monday.

In evidence yesterday, Mr O'Brien said he was living in Crumlin Park at the time. He was friendly with Johnston and Hempenstall, knew Kenny, but had never met Zambra.

He had also never met the deceased, who was known as Champers, he said.

A few days before the shooting, Johnston and Hempenstall asked if he wanted a bit of work. He understood this to mean "getting involved in something dodgy, bogie", and he said yes.

He said that he and Hempenstall then met Zambra, who said that John Carroll had to be shot by the weekend.

"Bernard Hempenstall was going to kill him. He owed Champers money," he testified. "Him and another guy owed him ¤140,000 between the two of them."

He gave evidence that Zambra, known as Git, said there would be €30,000 when 'the job' was finished.

"He made the offer to Bernard Hempenstall," he said.

He said that Hempenstall told him that Carroll would be shot in McCauley's pub in Rialto.

"He was going to have him there first and then he was going to leave," Mr O'Brien said.

"Bernard was going to shoot him. He said he had a gun parked up in a ... jeep he had parked in Harold's Cross," he said.

Mr O'Brien was to collect the gun from the jeep's glove compartment and burn the jeep "after the job was done", he said.

"Christopher Zambra told me to do that," he said.

He added that they had first intended to use the jeep for the shooting, but then a motorbike was bought for about €2,250. No money changed hands when Mr O'Brien and Johnston collected the Yamaha R1, a helmet and a pair of gloves.

Mr O'Brien said that he, Zambra and Hempenstall met again the day before the shooting.

"Damien Johnston was going to drive the bike and Hempo was going to shoot him. There was going to be a decoy, a bomb or something put under a car in Raleigh Square," he explained. "I just had to burn the bike or burn the jeep out."

Mr O'Brien said that he was arrested for public order on the evening of the killing. There were messages on his phone when released from custody.

"Hempo said he couldn't do the shooting, that John Carroll wanted to go for a drink with him somewhere else," he said. "Git said he'd have someone else to stand in for him."